1. Field of Invention
The present invention generally relates to an illumination system comprising a radiation source and a fluorescent material comprising an amber to red phosphor. The invention also relates to an amber to red phosphor for use in an illumination system. More particularly, the invention relates to an illumination system comprising an electroluminescent semiconductor device as radiation source and a fluorescent material comprising a phosphor for the generation of specific, white or colored light, including yellow, amber and red light. In this illumination system white or colored light is generated by luminescent down conversion and additive color mixing based on an ultraviolet or blue primary radiation. A solid-state light-emitting diode as a source of primary radiation is especially contemplated.
2. Description of Related Art
Various illumination systems for vehicular and signaling usage are known. Vehicles include a number of different components and assemblies that have an illuminator or a signal lamp associated therewith. Great interest has been shown in the use of electroluminescent semiconductor devices, such as solid-state light emitting diodes (LEDs), as illuminators and signal indicators because they offer many potential advantages as compared to other conventional low voltage light sources. Other light sources suffer from deficiencies, including relative inefficiency, such as is the case with conventional tungsten incandescent lamps; high operating voltages, such as is the case with fluorescent and gas discharge lamps; or susceptibility to damage, such as is the case with incandescent lamps.
Accordingly, these alternate light sources are not optimal for vehicular applications where only limited power or low voltage is available, or where high voltage is unacceptable for safety reasons, or in applications where there is significant shock or vibration. LEDs on the other hand are highly shock resistant, and therefore provide significant advantages over incandescent and fluorescent bulbs, which can shatter when subjected to mechanical or thermal shock. LEDs also possess operating lifetimes from 200,000 hours to 1,000,000 hours, as compared to the typical 1,000 to 2,000 hours for incandescent lamps or 5,000 to 10,000 hours for fluorescent lamps.
Current yellow, amber or red traffic and vehicular lights comprising an electroluminescent semiconductor device, rely on direct excitation of aluminum gallium indium phosphide (AlGaInP) LED chips for generation of colored yellow, amber or red light.
A drawback of AlInGaP LEDs is the quenching of the light emission with increasing temperature. Their light output drops by more than 40% if the temperature is raised from room temperature to 100° C. At the same time the spectrum shifts, e.g. from 617 nm to 623 nm, which reduces the luminous efficacy further. Therefore, there is a strong demand by the automotive industry for yellow to red LEDs with a much smaller dependence of the light yield and emission spectrum on temperature.
One presently discussed solution for generation of yellow, amber or red light is the application of white LEDs and an appropriate colour filter, since the AlInGaN chips applied in white LEDs show much less thermal quenching. In addition, the spectral shift of white LEDs with temperature is less severe due to the application of the YAG:Ce phosphor. However, the major drawback of this concept is the low efficiency due to the fact that the present white LEDs emit only a few percent orange to red light and most of the white LED spectrum is cut off.
Another approach is known, e.g. from U.S. Pat. No. 6,649,946 wherein a light source for generating of yellow to red light by using a yellow-to-red-emitting phosphor is disclosed. Said phosphor has a host lattice of the nitridosilicate type MxSiyNz:Eu, wherein M is at least one of an alkaline earth metal chosen from the group Ca, Sr, Ba, Zn and wherein z=⅔x+ 4/3y. The phosphor can be used to create a highly stable red or orange or yellow emitting LED which may be based on a primary light source (preferably InGaN-chip) of peak emission around 380 to 480 nm whose light is fully converted by a nitride phosphor of the inventive type rare-earth activated silicon nitrides doped with Eu. These LEDs show higher efficiency and improved stability compared to well-known commercial LEDs with direct excitation of yellow to red colors.
Yet, a recent evaluation of the chromaticity requirements for traffic signs has indicated that the red color range of vehicular and traffic signs should include a longer-wavelength cut-off to ensure detection of the signal by color vision deficient drivers.
Therefore, there is a need to provide an illumination system comprising phosphors that are excitable by a radiation source of the near UV-to-blue range and emit in the visible amber to deep-red range.